OP249

Rev. F | Page 15 of 20

DAC OUTPUT AMPLIFIER

Unity-gain stability, a low offset voltage of 300 μV typical, and a

fast settling time of 870 ns to 0.01%, makes the OP249 an ideal

amplifier for fast DACs.

For CMOS DAC applications, the low offset voltage of the

OP249 results in excellent linearity performance. CMOS DACs,

such as the PM7545, typically have a code-dependent output

resistance variation between 11 kΩ and 33 kΩ. The change in

output resistance, in conjunction with the 11 kΩ feedback resistor,

results in a noise gain change, which causes variations in the

offset error, increasing linearity errors. The OP249 features

low offset voltage error, minimizing this effect and maintaining

12-bit linearity performance over the full-scale range of the

converter.

Because the DAC output capacitance appears at the inputs of

the op amp, it is essential that the amplifier be adequately

compensated. Compensation increases the phase margin and

ensures an optimal overall settling response. The required lead

compensation is achieved with Capacitor C in Figure 48.

+15V

PM7545

12

500Ω

75Ω

AGND

DGND

DATA INPUT

REFERENCE

0.1µF

19

18

20

3

C

33pF

–15V

1

23

2

8

4

0.1µF

0.1µF

1

1/2

OP249

Figure 47. Fast Settling and Low Offset Error of the OP249 Enhances CMOS DAC Performance—Unipolar Operation

+15V

PM7545

500Ω

75Ω

AGND

DGND

12

DATA INPUT

0.1µF

–15V

1/2

OP249

1/2

OP249

7

5

6

4

R5

10kΩ

1%

R4

20kΩ

1%

R3

10kΩ

1%

1

8

2

3

1

2

3

0.1µF

C

33pF

0.1µF

REFERENCE

18

20

19

Figure 48. Fast Settling and Low Offset Error of the OP249 Enhances CMOS DAC Performance—Bipolar Operation